In terms of actual numbers I have no idea. But consider this. Your innermost cog has to be somewhere to the right of the centerline of the hub or the wheel will collapse. The centerline through the single chain ring, or the bisection of the distance between double rings, or the center ring of a triple, has to bisect the cassette. So increasing the width of the axle to maintain the integrity of the rear wheel constrains the reduction of Q-factor. Also, in extreme cases, the width of the rear tire also plays into this. The innermost chain line has to clear the rear tire.
Then there's the chain angle that stresses the chain, enhances friction, and can cause the chain to rub against the next larger cog. Chain angle increases as chain stay length decreases.